Honeycomb structure and method for its manufacture

ABSTRACT

A honeycomb structure having a large number of channels through along the axial direction and being defined by partition walls. A honeycomb structure containing a refractory particle to be an aggregate and metallic silicon and being porous. This honeycomb structure can be suitably used under high SV conditions as a filter for purifying automobile exhaust gas by treatment such as clogging or catalyst supporting.

TECHNICAL FIELD

The present invention relates to a honeycomb structure used for filtersand catalyst carriers or the like for purifying exhaust gases ofautomobiles.

BACKGROUND ART

Porous honeycomb structures have been widely used as filters forcollecting and removing granular material contained in dust-bearingfluids like diesel engine exhaust gas or as catalyst supporters forsupporting catalyst components for purifying exhaust gases. Also, it iswell known that refractory particles such as a silicon carbide (SiC)particle are used as composition material for such a honeycombstructure.

As a specifically relating art, for example, JP-A-6-182228 discloses aporous silicon carbide catalyst carrier of a honeycomb structureobtained by a preparation method that involves forming in a desiredshape a silicon carbide powder having a given specific surface area aswell as containing impurities, used as the starting material, drying andsubsequently firing in a temperature range of 1600 to 2200° C.

On the other hand, JP-A-61-26550 discloses a method of manufacturing arefractory bearing vitrifying material that features adding vitrifyingmaterial to a readily oxidized material or a refractory compositioncontaining a readily oxidized material, mixing the resultant materialwith a binding agent, kneading and molding, and subsequently bareburning a molded body thus molded in a furnace under a non-oxidizingatmosphere. JP-A-8-165171 discloses a silicon carbide molded articlemolded subsequently to the addition of an organic binder and aninorganic binder of a clay mineral base, glass base, or lithium silicatebase to a silicon carbide powder.

In addition, JP-A-6-182228 above presents as a conventional method formanufacturing a porous silicon carbide sintered compact a manufacturingmethod that includes adding a binding material such as vitrified flux orclay to carbonate particles to be an aggregate, molding, andsubsequently sintering and hardening the resulting molded body at thetemperature at which the aforementioned binding material.

Furthermore, JP-B-61-13845 and JP-B-61-13846 disclose a refractoryparticle graded to a specified particle size, comprising silica sand,ceramic pulverized substances, metal oxides such as Al₂O₃, TiO₂, ZrO₂,silicon carbide, nitrides, borides, or other refractory materials, and asuitable average particle size of a refractory particle, a particle sizedistribution of a refractory particle, the porosity of a tubular body,the average pore diameter of a tubular body, the pore volume of atubular body, and the partition wall thickness of a tubular body, andthe like, with respect to a high temperature ceramic filter formed in aporous, bottomed tubular shape with a refractory bonding material suchas liquid glass, flit, glaze, and the like.

Additionally, JP-B-8-13706 discloses a silicon carbide/metallic siliconcomposite having a structure integratedly joined via metallic siliconand a process for manufacturing the aforementioned composite usingsilicon carbide and metallic silicon formed by heat treating siliconaccumulated biomass under a argon or nitrogen atmosphere.

With a burned form (necking) by means of the recrystallization ofsilicon carbide powder itself, disclosed in JP-A-6-182228 as describedabove, the silicon carbide component is evaporated from the surfaces ofthe silicon carbide particles, and the evaporated component is condensedin the contact portions (neck portions) between particles, and thus theneck portions grow to yield the bonded state. However, evaporation ofthe silicon carbide requires a very high firing temperature, which leadsto a high cost. Also, a material of a high thermal expansion coefficientneeds to be fired at a high temperature. These create the problem ofdecreasing the firing process yield.

In addition, production of a filter of a high porosity, particularlywith a porosity of 50% or more, by firing via recrystallization of theaforementioned silicon carbide itself, leads to inhibition of the growthof the neck portion due to insufficient performance of the sinteringmechanism, thereby creating the problem of decreasing the strength ofthe filter.

Furthermore, the aforementioned material, with a very high thermalconductivity of 30 W/mK or more, has an advantage in suppressing a localheat evolution. However, for example, when a filter is used that has asystem of oxidizing and combusting a particulate with a catalyst carriedto continuously perform reproduction, it requires much time to raise thetemperature of the carrier because of a small amount of particulateaccumulated and an easy emission of heat. Therefore, it needs time toincrease the temperature to a temperature at which the catalystfunctions, which also creates the problem of producing cinders of theparticulate resulting in a decrease in reproduction efficiency.

An approach to bonding a raw material of a silicon carbide via glassmaterial, disclosed in JP-A-61-26550 and JP-A-6-182228, can handle thematter using a low firing temperature of 1,000 to 1,400° C. However, forexample, use of a sintered body prepared by this method as a materialfor a diesel particulate filter (DPF) for removing a particulatecontained in exhaust gases discharged from a diesel engine, causes theproblem of locally generated heat due to a small thermal conductivitywhen combusting particulates collected and accumulated on the filter.

Moreover, while the filters disclosed in JP-B-61-13845 and JP-B-61-13846are porous, they are a bottomed cylinder with a thick partition wall of5 to 20 mm and thus cannot apply to conditions of a high space velocity(SV) like in a filter for purifying automobile exhaust gas.

In addition, according to a composite and the manufacturing methodthereof indicated in JP-B-8-13706, the composite can be made to beporous; however, it is not easy to ensure a sufficient porosity when thecomposite is utilized as a filter, and particularly it is difficult touse the composite as a filter for collecting and removing granularmaterials contained in a dust-bearing fluid like diesel engine exhaustgas.

The present invention has been made taking into consideration theseconventional circumstances, and the object of the invention is toprovide a honeycomb structure which can be costlessly produced at arelatively low firingburning temperature while containing refractoryparticles such as a silicon carbide particle and which is sufficientlyporous, have a high specific surface area and can be suitably used as afilter for purifying automobile exhaust gas under high SV conditions bymeans of treatment of clogging, catalyst supporting, etc. and themanufacturing method thereof.

DISCLOSURE OF THE INVENTION

According to the present invention, there is provided a honeycombstructure having a large number of channels through along the axialdirection and defined by partition walls, characterized in that thestructure includes refractory particles and metallic silicon that forman aggregate and is porous.

According to the present invention, there is also provided a method formanufacturing a honeycomb structure, characterized in that the methodinvolves adding metallic silicon and an organic binder to a raw materialof refractory particles, admixing and kneading it, forming the resultingbody for ceramics into a honeycomb shape, calcining the obtained moldedbody to remove the organic binder in the molded body, and subsequentlyfiring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of plotting the porosity (%), strength (MPa) andthermal conductivity (W/mK) versus the amount of metallic Si powderformulated.

FIG. 2 is a microscopic photo of a crystal structure of a siliconcarbide sintered body prepared in Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

As discussed above, a honeycomb structure of the present inventioncontains refractory particles as well as metallic silicon for bondingthe refractory particles, and therefore can be fired at a relatively lowfiring temperature during its production. Accordingly, the productioncost can be cut and the yield can be improved. In addition, utilizationof metallic silicon for bonding refractory particles leads to its highthermal conductivity as compared with a conventional structure usingvitreous material for refractory particle bonding. Hence, for example,when the structure is employed in a DPF, it does not produce local heatgeneration resulting in damage of the filter even though the depositedparticulates are burned for filter reuse. Furthermore, the presentinvention is not a bottomed cylinder of a thick wall that is disclosedin JP-B-61-13845 and JP-B-61-13846, but is a porous honeycomb structure,and thus can be used under high SV conditions as a filter for purifyingautomobile exhaust gas, a catalyst support, or the like.

Additionally, in a honeycomb structure of the present invention,refractory particles constituting the honeycomb structure preferablyhave a structure wherein the particles are bonded together by means ofmetallic silicon in part of the particle surface thereof. FIG. 2 is amicroscopic photo that shows a honeycomb structure concerning thepresent invention and a crystalline structure of a silicon carbidesintered body. In the figure, the white part indicates metallic silicon10, the gray part a silicon carbide particle 11, the black part a pore12. As is shown, the silicon carbide particles 11 of a refractoryparticle are bonded to each other in part of the particle surface by themetallic silicon 10. In addition, the method for manufacturing a siliconcarbide sintered body as illustrated in FIG. 2 will be discussed later.

The above structure is formed without using metallic silicon more thannecessary, and thus the densification due to merging of metallic siliconitself caused in the burning process can be restrained. Accordingly,sufficient porosity is ensured to keep the pressure loss low when thestructure is used as a filter. Furthermore, the structure has a highthermal conductivity as well, and so, for example, when it is employedas a DPF or the like to collect and remove particulates contained inexhaust gas discharged from a diesel engine, a high porosity issufficiently guaranteed and also a local evolution of heat leading tothe damage of the filter is not effected because of having a highthermal conductivity even if a deposited particulate is combusted forfilter reuse.

A honeycomb structure of the present invention preferably has a thermalconductivity of 5 W/mK or more, in terms of avoiding a local generationof heat as discussed above.

In addition, a honeycomb structure of the present invention preferablyhas a structure wherein refractory particles are bonded together viametallic silicon with the shape of the raw material particle maintainedin its microstructure. When a honeycomb structure of the presentinvention is used as a filter for collecting and removing granularmaterial contained in a dust-bearing fluid, the porosity is preferablywithin the range of 30 to 90%. When the porosity of the honeycombstructure is below 30%, the speed of filtration is not sufficient; thestrength as a structure is low when the porosity exceeds 90%.Furthermore, when the structure is used in an application in which apressure loss in a filter for purifying automobile exhaust gas, etc. maybe effected, the porosity is preferably made to be 40% or more.

Moreover, when the honeycomb structure is utilized as a filter that isused with the pressure loss maintained low, including a filter systemthat continuously combusting particulates by use of a catalyst assupporter, preferably the porosity is from 50 to 90% and the thermalconductivity from 5 to 30 W/mK, more preferably the porosity is from 50to 80% and the thermal conductivity from 7 to 28 W/mK, and particularlypreferably the porosity is from 53 to 70% and the thermal conductivityfrom 9 to 25 W/mK.

In a honeycomb structure used as a filter system wherein a catalyst issupported, a porosity needs to be set high in advance inasmuch ascarrying a catalyst increases the pressure loss. Therefore, a porosityof less than 50% is not preferable in that the filter system renders thepressure loss large. On the other hand, a porosity exceeding 90% is notpreferable due to lack of strength as a structure.

Further, in a honeycomb structure used in a filter of the aforementionedsystem, a local stress in a filter generated by a nonuniform temperaturedistribution caused by a local heat generation needs to be restrained tooccur. Thus, a thermal conductivity of below 5 W/mK is difficult toeffectively suppress the local heat generation. On the other hand, whenthe thermal conductivity exceeds 30 W/mK, the effect of heat radiationbecomes large and the amount of particulates accumulated is small. As aconsequence, it requires much time to raise the temperature to atemperature at which the catalyst functions due to resistance totemperature rising, and also cinders of the particulate are generated,which lowers the efficiency of filter reproduction. Hence, the case isnot preferable.

In addition, catalysts supported in a filter concerning the presentinvention refer to catalysts used to combust a particulate and decomposeNOx, and more particularly include noble metals such as platinum,palladium, rhodium, iridium and silver and oxides such as alumina,zirconia, titania, ceria and iron oxides, but the present invention isby no means limited to these species.

Similarly, when a honeycomb structure of the present invention is usedas a filter, the average pore diameter of the honeycomb structure ispreferably determined on the basis of the target of filtration. Forexample, if the structure is used as a DPF for collecting and removingparticulates contained in exhaust gas discharged from a diesel engine,the average pore diameter is preferably made to be within the range of 2to 50. When the average pore diameter is below 2 μm, the pressure lossis remarkably raised even via a small amount of particulate accumulated;conversely, when the diameter is over 50 μm, a particulate passesthrough without being affected. Thus, the case is not preferable.

A suitable content of metallic silicon in a honeycomb structure of thepresent invention is preferably within the range of 5 to 50% by weightwith respect to the total amount of the refractory particle and themetallic silicon, although it varies with the particle size and particleshape of the refractory particle, and more preferably within the rangeof 15 to 40% by weight. When the content is less than 5% by weight,neighboring refractory particles insufficiently bond to each other viametallic silicon due to lack of binding material, not only leading to adecrease in the thermal conductivity, but making it difficult to obtainthe strength of allowing a thin-walled structure like a honeycombstructure to be maintained. Conversely, if the content exceeds 50% byweight, metallic silicon is present in an amount more than the extent ofsuitably causing refractory particles to bond to each other, which inturn effects excessive contraction of the honeycomb structure (sinteredbody) by burning, leading to detrimental effects such as a porositydecrease and average pore size contraction. Thus, the case is notpreferable.

The thickness of a partition wall defining the channel pore (cell) of ahoneycomb structure is preferably made to be 4 mil or more (102 μm ormore). If the thickness of a partition wall is less than 4 mil (102 μm),the strength as a structure is insufficient. In addition, the strengthand the porosity are closely related, and for a honeycomb structure ofthe present invention, when the thickness of a partition wall is set ina manner such that the thickness of the partition wall and the porositysatisfies the relation below, it is shown that a necessary strength ispreferably obtained:

Thickness of partition wall (μm)≧porosity (%)×4.

Furthermore, the thickness of a partition wall is more preferably set sothat the thickness of the partition wall and the porosity satisfies thefollowing relation, to obtain a sufficient strength:

Thickness of partition wall (μm)≧porosity (%)×5.

On the other hand, when the structure is used as filters such as DPF,the thickness of the partition wall is preferably 50 mil or less (1270μm or less). When the thickness of the partition wall exceeds 50 mil(1270 μm), concerns rise about lack of a filter speed and a rise inpressure loss. In addition, these are also closely related to theporosity, and problems can be avoided by setting the thickness of thepartition wall in such a way that the thickness of the partition walland the porosity satisfy the following relation:

Thickness of partition wall (μm)≦porosity (%)×20.

The cell density of a honeycomb structure is preferably in the range of5 to 1000 cells/square inch (0.7 to 155 cells/cm²). When the celldensity is less than 5 cells/square inch (0.7 cell/cm²), the honeycombstructure is low in strength; when the structure is used as a filter,the filtration area is also in lack. Conversely, if the density exceeds1000 cells/square inch (155 cell/cm²), a rise in pressure loss does notpreferably result.

Now, the method for manufacturing a honeycomb structure of the presentinvention will be described. When producing a honeycomb structure of thepresent invention, first, metallic silicon and an organic binder areadded to the raw material of a refractory particle, and then theresulting material is admixed and kneaded to obtain a body for ceramicsfor molding.

The kind of refractory particles to be used are not particularlylimited, but suitable examples for use include oxides such as Al₂O₃,ZrO₂ and Y₂O₃, carbides such as SiC, nitrides such as Si₃N₄ and AlN, andothers including mullite. For example, with applications such as DPFsthat is often exposed to elevated temperatures when an accumulatedparticulate is treated by combustion, SiC or the like, having a highheat resistance, is suitably used. In addition, although raw materialsused for refractory particles and metallic silicon sometimes containtrace quantities of impurities such as Fe, Al and Ca, they can bedirectly used or may be utilized after purification by chemicaltreatment such as purifying by chemicals.

The average particle diameter of raw materials of refractory particlesis preferably 2 to 4 times the average pore diameter of a honeycombstructure (burned body) finally obtained by the present manufacturingmethod. A honeycomb structure obtained by the present manufacturingmethod exhibits a comparatively low firing temperature, and thus theparticle shape and the particle diameter are maintained roughly untilthe end of firing. Therefore, if the aforementioned ratio is a factor ofless than 2, the particle diameter is too small relative to the porediameter; as a result, groups of small refractory particles are bondedin a long and narrow shape by means of metallic silicon to form a largepore, and thus a strength cannot be provided high enough to keep athin-walled structure like a honeycomb structure.

Also, for example, if a refractory particle is a SiC particle,recrystallized SiC that has been applied to a conventional poroushoneycomb structure requires a particle diameter of the raw material ofan aggregate almost equivalent to a desired pore diameter, while withSiC particles bonded together via metallic silicon like a honeycombstructure of the present invention, the particle diameter may be 2 timesthe pore diameter, and so, when the same pore diameter is to beobtained, a rough material relative to recrystallized SiC, i.e., aninexpensive material, can be utilized, leading to a large cost merit aswell.

Inversely, if the aforementioned ratio exceeds 4 times, the particlediameter of a refractory particle used for a desired pore diameter istoo large, and a desired pore in its space is difficult to obtain eventhough the refractory particles are closely packed in the stage ofmolding, and further a decrease in porosity in a filter applicationresults. Thus, the case is not preferable.

Metallic silicon melts during burning to moisten the surface ofrefractory particles and serve to bond the particles to each other. Asuitable amount of the particle to be added, although depending on theparticle diameter and the particle shape of refractory particles, ispreferably within the range of 5 to 50% by weight based on the totalamount of the refractory particle and the metallic silicon. When thecontent is less than 5% by weight, it causes a shortage and makes itdifficult to obtain the strength of allowing a thin-walled structurelike a honeycomb structure to be maintained. Inversely, if the contentexceeds 50% by weight, metallic silicon is present in an amount morethan the extent of suitably causing refractory particles to bond to eachother, and thus it leads to detrimental effects such as a porositydecrease and average pore size contraction.

The average diameter of metallic silicon is preferably 50% or less ofthe average diameter of a refractory particle, or an aggregate. Metallicsilicon melts by firing and collects, and moves while sticking to therefractory particles, and so, when the particle diameter exceeds 50% ofthe particle diameter of a refractory particle, space occupied by themetallic silicon particles becomes a large void during molding andremains, which leads to a decrease in strength, or causes decline infilter efficiency (filter leakage) when the structure is utilized as afilter.

In addition, generally, during extrusion of a honeycomb structure,mixing of 2 species or more of raw material powders having differentparticle sizes rather leads to smooth extrusion. Accordingly, also inorder to obtain an appropriate structure as a porous body, the averageparticle diameter of metallic silicon is preferably made to be 50% orless of the average particle diameter of a refractory particle, or anaggregate.

In order to smoothly extrude in a honeycomb shape a body for ceramicsformulated with metallic silicon and, as necessary, a pore formingagent, etc. using a refractory particle as aggregate, at least onespecies of organic binders is preferably added as a molding adjuvant inan amount of 2% by weight or more as a superaddition to the total amountof the primary raw materials (raw material of a refractory particle andmetallic silicon). However, addition exceeding 30% by weight is notpreferable inasmuch as it causes an overly high porosity aftercalcination, resulting in a lack in strength.

Furthermore, when a honeycomb structure with a partition wall thicknessof 20 mil (508 μm) or less is extruded, the molding adjuvant ispreferably added in the range of 4 to 20% by weight. The amount ofaddition being 4% by weight or less makes it difficult to extrude insuch a thin wall; conversely, when the amount exceeds 20% by weight, itsshape after extrusion is difficult to maintain.

When a honeycomb structure is utilized as a filter, a pore forming agentmay be added during preparation of a body for ceramics for the purposeof enhancing the porosity. The amount of pore forming agent to be addedis preferably 30% by weight or less as a superaddition to the totalamount of the main raw materials (raw material of a refractory particleand metallic silicon). An amount of addition exceeding 30% by weightleads to lack in strength due to the porosity being extremely high.

In addition, even when there is obtained a honeycomb structure with ahigh porosity of 50% or more, a pore forming agent is preferably added.In this case, an appropriate selection of the kind and average particlediameter of a pore forming agent to be used permits the production of ahoneycomb structure of a high porosity, with the pore size distributioncontrolled. That is to say, while voids among particles of refractoryparticles, or aggregates, become pores in the present invention, theaddition of an appropriate amount of a pore forming agent with aparticle diameter being 1.2 to 4 times the average particle diameter ofa refractory particle, or an aggregate, can lead to the production of ahoneycomb structure of a high porosity having pore size distributionscomprising two pore size distributions of voids among particles ofrefractory particles and burned traces of a pore forming agent.Therefore, an appropriate selection of the particle diameters ofrefractory particles and a pore forming agent allows flexible materialdesigning with a necessary pore size distribution.

On the other hand, in order to prepare a honeycomb structure of a largepore size, a body for ceramics can be smoothly extruded during extrusionby the addition of an appropriate amount of a pore forming agent havinga particle diameter being 0.5 times or less the average particlediameter of a refractory particle even when a refractory particle of alarge particle diameter or metallic silicon is employed. Therefore, ahoneycomb structure of a high porosity can be produced without loweringmoldability.

The kind of a pore forming agent to be used is not particularly limited,and more specifically examples include graphite, wheat flour, starch,phenol resin, polymethyl methacrylate, polyethylene, and polyethyleneterephthalate. A pore forming agent may be used in combination of aspecies, or two species or more, depending on its purpose.

A body for ceramics obtained by admixing and kneading the aforementionedraw material using the usual method is molded in a desired honeycombshape by means of the extrusion method. Thereafter, the molded body thusobtained is calcined to remove an organic binder (degrease) contained inthe molded body and then burned. Calcination is preferably performed ata temperature lower than the temperature at which metallic siliconmelts. More specifically, it may be kept temporarily at a specifictemperature from about 150 to 700° C., or it may be carried out byrendering the rate of temperature rise to be 50° C./hr or less in agiven temperature range.

A method of temporarily keeping the temperature at a given temperaturecan select the keeping at one temperature or at a plurality oftemperatures. In the case of keeping at a plurality of temperatures, theretention times may be the same or different to each other. In addition,a method of decreasing the rate of temperature rise may also select thedecreasing only in a given temperature range or in a plurality oftemperature ranges. Moreover, for the plurality of ranges, the rates arethe same or different to each other.

The atmosphere for calcination may be an oxidation atmosphere, but whenorganic binders are contained in a molded body in quantity, theysometimes vigorously burn due to oxygen to cause the temperature of themolded body to be rapidly raised. Thus, a method is also preferable thatrestrains the abnormal temperature rise of the molded body by carryingout calcination under an inert atmosphere of N₂, Ar, etc. Suppression ofthis abnormal temperature rise is an important control when a rawmaterial of a large thermal expansion coefficient (weak in thermalimpact) is employed. When organic binders are added, for example, in anamount of 20% by weight or more as a superaddition to the amount ofprimary raw materials, it is preferable to be calcined under theaforementioned inert atmosphere. In addition, besides the case of arefractory particle being a SiC particle, even when oxidation may occurat a high temperature, it is preferable to restrain the oxidation of amolded body by performing calcination under the aforementioned inertatmosphere at least at a temperature at which oxidation starts or at atemperature higher than the temperature.

Burning subsequent to calcination may be performed in the same furnaceor in a separate furnace as a separate step or in the same furnace as acontinuous step. When calcination and burning are carried out underdifferent atmospheres, although the former method is also preferable,the latter method is preferable from the viewpoint of the total burningtime and the operation cost of the furnace as well.

In order to obtain a structure wherein refractory particles are bondedvia metallic silicon, metallic silicon needs to be softened. Because themelting point of metallic silicon is 1410° C., the burning temperatureduring burning is preferably 1400° C. or higher. A further suitableburning temperature is determined from microstructures andcharacteristic values. It should be noted that the burning temperatureis appropriately from 1400 to 1800° C. in as much as bonding viametallic silicon becomes difficult at temperatures exceeding 1800° C.due to the advance of evaporation of metallic silicon.

In addition, the manufacturing method using the recrystallizing methoddisclosed in JP-A-6-182228 above can provide a sintered body of a highthermal conductivity because of silicon carbide particles being bondedto each other. However, since sintering is brought by a mechanism ofevaporation/condensation as discussed above, in order to evaporate thesilicon carbide, a firing temperature higher than that of themanufacturing method of the present invention is required, and a hightemperature of at least 1800° C., normally 2000° C. or higher isrequired to obtain a practically usable silicon carbide burned body.

The atmosphere of firing is preferably selected according to the kind ofa refractory particle. For example, particles of carbides including SiC,particles of nitrides representatively including Si₃N₄ and AlN, whichmay be oxidized at elevated temperatures, are preferably placed atnon-oxidizing atmospheres such as N₂ and Ar at least in the temperaturerange of a temperature at which oxidation starts or higher.

Hereinafter, the present invention will be discussed in more detail bymeans of examples, but the present invention is by no means limited tothese examples.

EXAMPLES 1 to 13, COMPARATIVE EXAMPLES 1 to 2

A SiC raw material powder having an average particle diameter and ametallic Si powder with an average particle diameter of 4 μm, as shownin Table 1, are formulated on the basis of the compositions indicated inthe table, and to 100 parts by weight of this resulting powder, 6 partsby weight of methylcellulose as an organic binder, 2.5 parts by weightof a surfactant and 24 parts by weight of water were added and thenuniformly admixed and kneaded to give a body for ceramics for molding.The body for ceramics thus obtained was formed by means of an extruderinto a honeycomb shape having an outer diameter of 45 mm, a length of120 mm, a partition wall thickness of 0.43 mm and a cell density of 100cells/square inch (16 cells/cm²). This honeycomb molded body wascalcined at 550° C. for 3 hours for degreasing, and then was burned for2 hours under a non-oxidation atmosphere at a burning temperatureindicated in Table 1 to prepare a silicon carbide burned body of aporous honeycomb structure. This burned body was subjected tomeasurements of the average pore diameter and the porosity using amercury porosimeter, the thermal conductivity with a laser flash method,and further the 4-point bending strength. The results are listed inTable 1. In addition, FIG. 2 shows a microscopic photo of the crystalstructure of the silicon carbide sintered body prepared in Example 1.Furthermore, FIG. 1 illustrates a graph plotting the porosity (%),strength (MPa) and thermal conductivity against the amount of metallicSi powder formulated (wt %). Additionally, X-ray diffraction determinedthe crystalline phase and confirmed that it consists of SiC and Si.

TABLE 1 Average Average particle particle Amount of diameter Amount ofdiameter SiC of metallic Average 4-Point of SiC powders metallic Sipowders Burning pore bending Thermal powders formulated Si powdersformulated temperature diameter Porosity strength conductivity (μm) (wt%) (μm) (wt %) (° C.) (μm) (%) (Mpa) (W/mk) Example 1 32.6 80 4 20 14509.0 49.0 20 21 Example 2 32.6 80 4 20 1600 10.0 44.0 25 20 Example 332.6 65 4 35 1450 12.0 45.0 25 25 Example 4 32.6 65 4 35 1600 13.0 42.028 26 Example 5 50.0 80 4 20 1450 11.6 45.0 20 21 Example 6 50.0 80 4 201600 13.5 49.0 22 20 Example 7 32.6 90 4 10 1450 9.0 45.0 16 15 Example8 32.6 85 4 15 1450 9.0 47.0 20 20 Example 9 32.6 80 12 20 1450 11.043.0 20 20 Example 10 32.6 80 30 20 1450 13.0 42.0 18 25 Example 11 32.670 4 30 1450 12.0 47.0 27 23 Example 12 32.6 60 4 40 1450 12.0 43.0 2328 Example 13 32.6 55 4 45 1450 14.0 40.0 20 30 Comparative 32.6 97 4 31450 8.0 45.0 3 3 Example 1 Comparative 32.6 45 4 55 1450 16.0 25.0 1823 Example 2

(Discussions)

There were found decreases in strength and thermal conductivity inComparative Example 1 and a decrease in porosity in Comparative Example2. On the other hand, Examples 1 to 13 concerning the present inventionindicate, for examples, sufficient numerical values for the porosity,strength and thermal conductivity required when the structure is used,for example, as a DPF for collecting and removing particulates containedin exhaust gas discharged from a diesel engine. Also, the graphillustrated in FIG. 1 shows that an appropriate amount of metallic Sipowder to be added lies within the range of 5 to 50% by weight withrespect to the total amount of the SiC raw material powder and themetallic Si powder. These results have confirmed an excellent effect ofthe present invention.

EXAMPLES 14 to 20

A SiC raw material powder and a metallic Si powder with an averageparticle diameter, as shown in Table 2, are formulated on the basis ofthe compositions indicated in the table, and further to 100 parts byweight of this resulting powder, the amount (parts by weight) indicatedin the table of polymethyl methacrylate as a pore forming agent, 8 partsby weight of methylcellulose as an organic binder, 2.5 parts by weightof a surfactant, and 28 parts by weight of water were added, and then ahoneycomb structure of a silicon carbide sintered body was prepared by asimilar method as in Examples 1 to 13. In addition, a firing temperatureof 1450° C. was used for every case. This sintered body was measured forthe average pore diameter and the porosity using a mercury porosimeterand for the thermal conductivity with a laser flash method.

TABLE 2 Average Average Average particle particle Amount of particleAmount of diameter Amount of diameter pore- diameter SiC of metallic Siof pore- forming Average of SiC powders metallic powders forming agentpore Thermal powders formulated Si powders formulated agent formulatedPorosity diameter conductivity (μm) (wt %) (μm) (wt %) (μm) (%) (%) (μm)(W/mk) Example 14 32.6 80 4 20 60 20 58.0 21.0 14 Example 15 32.6 75 425 12 14 53.0 13.0 25 Example 16 47.0 85 12 15 12 20 60.0 18.0 12Example 17 47.0 80 12 20 12 20 58.0 15.0 16 Example 18 68.0 85 12 15 3020 55.0 30.0 18 Example 19 68.0 90 12 10 60 25 66.0 40.0 10 Example 2032.6 80 4 20 60 30 70.0 25.0 9

(Discussion)

Table 2 shows that a honeycomb structure of the present invention, forexample, indicates sufficient numerical values of the porosity, thermalconductivity and average pore diameter required when the structure isused as a filter having a catalyst carried or the like for purifyingautomobile exhaust gas. In addition, even when the particle diameter ofa SiC powder, or an aggregate, is made large (Examples 16 to 19), ahoneycomb structure was successfully produced without molding defects byadjusting the particle diameter and formulation amount of a pore formingagent. This has confirmed the excellent effect of the present invention.

INDUSTRIAL APPLICABILITY

As discussed above, a honeycomb structure of the present invention,while containing refractory particles such as a silicon carbideparticle, can be sintered at comparatively low burning temperatureduring the production thereof. Therefore, the production cost can besuppressed and the yield can be improved, that is, the product can beprovided at a low price. In addition, the structure has a high thermalconductivity relative to a conventional structure wherein refractoryparticles are bonded through the use of glass material, and so, evenwhen it is, for example, used for a DPF, or even if an accumulatedparticulate is combusted for filter reproduction, a local heat evolutionthat damages the filter does not occur. Moreover, the structure has aporosity and a thermal conductivity in specified numerical ranges, andis-a porous honeycomb structure with a low pressure loss, and thereforecan be suitably used as a filter having a catalyst supported or the likefor purifying automobile exhaust gas even under high SV conditions.

1. A honeycomb structure having a large number of channels through alongthe axial direction and defined by partition walls, wherein thestructure is porous and includes refractory particles and metallicsilicon, wherein the refractory particles are bonded via said metallicsilicon in part of the refractory particle surface as a non-continuouscoating, thereby forming an aggregate.
 2. The honeycomb structureaccording to claim 1, wherein the structure has a thermal conductivityof 5 W/mK or more.
 3. The honeycomb structure according to claim 1,wherein said refractory particles keeping the raw material particleshape are bonded via said metallic silicon.
 4. The honeycomb structureaccording to claim 1, wherein said refractory particle is a siliconcarbide particle.
 5. The honeycomb structure according to claim 1,wherein the structure is used as a filter for collecting and removinggranular materials contained in a dust-bearing fluid.
 6. The honeycombstructure according to claim 1, wherein the porosity is within the rangeof 30 to 90%.
 7. The honeycomb structure according to claim 1, whereinthe average pore diameter is within the range of 2 to 50 μm.
 8. Thehoneycomb structure according to claim 1, wherein the porosity is withinthe range of 50 to 90% and the thermal conductivity is within the rangeof 5 to 30 W/mK.
 9. The honeycomb structure according to claim 1,wherein the content of said metallic silicon is within the range of 5 to50% by weight with respect to the total amount of the raw material ofsaid refractory particle and the metallic silicon.
 10. The honeycombstructure according to claim 1, wherein said partition wall thickness isfrom 102 to 1270 μm.
 11. The honeycomb structure according to claim 1,wherein said partition wall thickness and the porosity of a honeycombstructure satisfy the relation: partition wall thickness (μm)≧porosity(%)×4.
 12. The honeycomb structure according to claim 1, wherein saidpartition wall thickness and the porosity of a honeycomb structuresatisfy the relation: partition wall thickness (μm)≧porosity (%)×5. 13.The honeycomb structure according to claim 1, wherein said partitionwall thickness and the porosity of a honeycomb structure satisfy therelation: partition wall thickness (νm)≧porosity (%)×20.
 14. Thehoneycomb structure according to claim 1, wherein the cell density is0.7 to 155 cells/cm².
 15. The honeycomb structure according to claim 1,wherein the metallic silicon has an average particle diameter that is50% or less of an average diameter of the refractory particle.
 16. Amethod for manufacturing a honeycomb structure having a large number ofchannels through along the axial direction and defined by partitionwalls, and the structure is porous and includes refractory particles andmetallic silicon that form an aggregate by bonding of the refractoryparticles via the metallic silicon in part of the refractory particlesurface as a non-continuous coating, wherein the method involves addingmetallic silicon in such a sufficient amount that the metallic siliconremains in a required amount even after burning and an organic binder tothe raw material of refractory particles, mixing and kneading it,forming the resulting body for ceramics into a honeycomb shape,calcining the obtained molded body to remove the organic binder in themolded body, and subsequently burning.
 17. The manufacturing methodaccording to claim 16, wherein the raw material of said refractoryparticle is the raw material of a silicon carbide particle.
 18. Themanufacturing method according to claim 16, wherein the average particlediameter of the raw material of said refractory particle is 2 to 4 timesthe average pore diameter of a honeycomb structure finally obtained. 19.The manufacturing method according to claim 16, wherein the amount ofsaid metallic silicon to be added is within the range of 5 to 50% byweight with respect to the total amount of the raw material of saidrefractory particle and the metallic silicon.
 20. The manufacturingmethod according to claim 16, wherein the average particle diameter ofsaid metallic silicon is 50% or less of the average particle diameter ofa refractory particle, or an aggregate.
 21. The manufacturing methodaccording to claim 16, wherein said organic binder is added within therange of 2 to 30% by weight as a superaddition to the total amount ofthe raw material of said refractory particle and the metallic silicon.22. The manufacturing method according to claim 16, wherein uponpreparing said body for ceramics, a pore forming agent is added in therange of 30% by weight as a super addition to the total amount of theraw material of said refractory particle and the metallic silicon. 23.The manufacturing method according to claim 16, wherein said calcinationof a molded body is carried out at a temperature lower than thetemperature at which said metallic silicon melts.
 24. The manufacturingmethod according to claim 16, wherein said burning is performed withinthe temperature range of 1400 to 1800° C.